You must explicitly test by disable/enable of AV to determine the impact on Thunderbird.
That is NO guarantee it will work well in any future version of AV or Thunderbird.Problems have been reported with: Also, several products have caused pop download errors or slowness if incoming mail is scanned  The following programs have generally been observed as working well with Thunderbird (but not proof that these products can't have problems): AVG can not scan SSL connections and requires you to set it up as a proxy and place the mail settings in AVG and pointers to the proxy in Thunderbird.AVG offers instructions here AVG causes intermittent Not Responding in Windows 10. Mc Afee has had major problems with Thunderbird over the years, causing crashes and poor performance, "Not Responding", "Unresponsive Script", and other issues.If none of the above helps and you suspect Mc Afee is the cause then you may need to remove Mc Afee using the Mc Afee Consumer Products Removal tool, and seek different antivirus software. Antivirus software can also cause bad or incorrect behavior, and crashes - consistently or randomly.The information here is anecdotal, that is users like you giving their personal experiences. So if you have a problem with antivirus software that is not listed here, please help other users by adding your own experience to the list. Warning: Your AV software has probably worked great for ages in previous versions of Thunderbird.
That is NO guarantee that it will work well in any future version of AV or Thunderbird.
When either AV or Thunderbird changes versions there can be interaction problems - even if for you Thunderbird is the only software affected, and even if your AV version did not change.
When either one changes version there can be interaction problems - even if for you Thunderbird is the only software affected, and even if your AV version did not change.
To test whether Antivirus or Anti-Malware software is causing a problem, you must disable the software temporarily or run Windows in safe mode.
Permanent solutions might require you to disable parts of the software, change to different software, or exclude Thunderbird profile folder locations from scanning.
The default profile location on Windows is %appdata%\thunderbird\profiles **NOTE, if you altered default profile location or Thunderbird accounts to put mail somewhere on disk other than the standard profile location, you will need to also exclude those nonstandard locations.